Potential drawbacks to known valves include limited ability to i) provide fine control over the flow restriction; ii) provide relatively linear control of the flow; iii) achieve fast response time from fully closed to fully open without excessive power consumption; iv) be easily controlled electronically; v) control flow over a very wide range of flow rates; vi) function without lubrication; vii) in ventilation applications involving blowers, enable diversion of airflow to an exhaust port when the valve is closed, and viii) control flows at very low pressures, such as in ventilation applications, due to the size of the orifice opening required.
Two types of known valves are globe valves and piston valves. These valves are common, but suffer from a few problems, including size, slow response time, and high restriction even for low pressure drops, which creates a high pressure drop at low flows. Specifically, for ventilator applications, the same valve cannot be used for adult patients as for neonates, because the flow ranges are very different and the resolution of control at low flow rates is low.
Other types of known valves are sliding valves and other rotating valves such as plug valves, ball valves, and butterfly valves. These valves suffer from slow response time and imprecise flow control. They are good for applications requiring a simple on/off flow control, but their flow restriction is non-linear as the valve opens and closes. They are thus unable to provide proportional flow control. Another problem with these valves is that, usually, in order to enable complete sealing they require lubrication for the moving parts.